1. Field of the Invention
Embodiments of the present invention relates to a hybrid cell in a mobile communication system, and more particularly to a method of camping on a hybrid cell, a method of checking an access priority, a hybrid cell Node B and a user equipment (UE).
2. Description of the Prior Art
As an important organization in the mobile communication field, 3rd Generation Partnership Project (3GPP) has greatly promoted the standardization development of the Third Generation (3G) mobile communication technology and has instituted series of communication system specifications. In order to meet the challenge of the broadband access technology and the requirement of increasing new services, 3GPP initiated the standardization of the 3G Long Term Evolution (LTE) at the end of 2004, which aims to further improve the spectrum efficiency and the performance of the user at the cell border, reduce the system delay and provide high speed mobile users with higher rate access services. In order to avoid market splitting caused by non-standard solutions of different manufacturers and to guarantee the multi-vendor provision environment of Home Evolved Node B (HeNB), 3GPP initiates a studyitem (SI) “Study on Home NodeB/eNodeB” in the TSGRAN#35 conference in March 2007, which aims to form a standard solution according to the typical application scenario of the HeNB. This SI is supported by three work groups RAN2, RAN3 and RAN4, which implement feasibility research in HeNB architecture, mobility, access control and radio frequency etc. With the development of 3G market and the expansion of user amount, the network operators propose requirements of how to provide high speed data service with low cost in the home environment and pay much attention to the new 3G product of HeNB.
Since HeNB is oriented to application scenarios such as home, small enterprises and the like, the HeNB may be owned by the user. Therefore, the HeNB system naturally needs to decide whether to provide service according to the user's identity, i.e. the HeNB system needs to have an admission control function. For example, a user make a contract with an operator to install a HeNB in his house and the user only wishes to provide services for family members and certain visitors. According to this requirement for the HeNB, 3GPP proposed the concept of Close Subscriber Group (CSG) to perform the admission control in Release 8 version. CSG refers to a group of contracted subscribers admitted to access one or more certain HeNB cells, wherein the subscribers are limited to access the one or more certain HeNB cells. The same UE may belong to multiple CSGs and each CSG is identified by one CSG identification (ID). The UE maintains a CSG ID list (admitted CSG ID list) of the CSGs it belongs to and the CSG cells corresponding to other CSG IDs outside this list are not admitted to access for this UE. Each CSG cell broadcasts the CSG ID of the cell and the members of the CSG identified by this CSG ID may be admitted to this cell.
In order to perform the admission control for the members in the CSG to access the CSG cell conveniently, the CSG cell needs to identify its access characteristic in the synchronization channel and broadcast information. In the defined Release 8 version, the specific definitions are as following.
—Physical Cell Identification Split (PCI split). Physical Cell Identification (PCI) is a sequence used for cell synchronization and is transmitted by the Node B in the downlink synchronization channel. Each cell has one PCI and when the user performs the cell search, he will first get the PCI and then is downlink synchronized with the cell according to the PCI. In order to make the user identify the CSG cell during the cell search, a part of PCIs, i.e. PCI split is specially split for the CSG cell in Release 8. The PCI split defines a range of PCIs, such as 0-50. If a cell is the CSG cell, the PCI within the range will be used; otherwise the PCI outside the range will be used.
—CSG Indicator. This indicator includes 1 bit information and is located in the System Information Block (SIB) 1 of the broadcast information, indicating whether a cell is a CSG cell. When the user reads this information and if its value is TRUE, it indicates that the cell is a CSG cell; otherwise, if the value is FALSE, it indicates the cell is a normal cell.
—CSG Identification (ID). In the discussion result of Release 8, this ID includes 27 bits and is also located in the SIB1, for uniquely identifying one CSG cell. As discussed above, when a user moves into the coverage of a CSG cell, after the synchronization processing and obtaining the CSG indicator, the present cell is identified as a CSG cell. Further, after comparing the CSG ID of the cell with the admitted CSG ID list (Whitelist) in its Subscriber Identification Module (SIM), it can be determined whether the user may access this CSG cell. Specifically, when the CSG ID of this cell is included in the Whitelist, the user may camp on or access this cell; otherwise, the user cannot camp on or access this cell.
The adoption of the HeNB may greatly improve the data transmission rate of home users and can improve the performance of the user at the cell border. However, the introduction of the HeNB may also bring other problems, such as the interference between the cellular cell and the HeNB and the unbalanced load between them. In order to solve these problems, 3GPP introduces a hybrid mode HeNB, i.e. the hybrid cell Node B in Release 9 version. The access right of the hybrid cell Node B is between those of the normal Node B and the CSG Node B. On one hand, it may provide the CSG user with the same performance service as that of the CSG cell; on the other hand, it may provide the non-CSG user with limited performance service in the case that the CSG user is not influenced. Because of the above characteristics, the hybrid cell has obtained a lot of attention. Currently, based on the above CSG cell access IDs, the access modes specific to the hybrid cell Node B are defined as following.
1) The PCI is selected outside the PCI split;
2) The CSG Indicator is given a value of False (such as 0 in binary);
3) The CSG ID includes 27 bit information, uniquely identifying the hybrid cell Node B.
When the UE obtains the above information in the synchronization channel and the broadcast information, the cell transmitting the information may be identified as a hybrid cell.
Although the hybrid cell Node B may solve some of interference problems and unbalanced load problems, its introduction also brings some new problems in the access procedure of the user.
FIG. 1 shows an initial access procedure of UE according to the prior art. Before the UE camping on the cell initiates a random access, an access priority check should be performed for the UE; after the UE passed the access priority check, the UE accesses the cell by the random access procedure; after the random access is successful, the default bearer is established and the IP address, QoS and etc. are negotiated; then the admission control procedure and scheduling procedure are performed.
Currently, the hybrid cell Node B allows the non-CSG user to camp on the cell. When the non-CSG user initiates a random access, since the hybrid cell Node B prefers to provide services for CSG users, this access request may be refused during the admission control procedure or the non-CSG user already in an established communication connection may be switched to another cell because he influences the CSG user's service. The consequence is that when the status of the hybrid cell Node B is not suitable for accessing the non-CSG user, on one hand, the initial access delay of the non-CSG user will be increased and his established communication connection will be influenced; on the other hand, the consumption of the time and frequency resources and pilot sequence resources for random access will be increased and non-necessary resource consumption will be brought.